The invention relates generally to the field of photovoltaic (PV) solar power systems, and more specifically to circuits for protecting bypass diodes from overheating under partial shading conditions, and protecting human personnel from arc flash hazards.
FIG. 1 is a high level block diagram of a conventional PV solar power system 10 including a plurality of PV segments 11 connected in series. Each PV segment 11 comprises a plurality of PV cells that are serially connected between a positive terminal 12 and a negative terminal 13. For example, a typical PV segment includes twenty four PV cells, and produces about 12V between 12 and 13 in full sunlight. So a typical string of twenty four to fifty one serially-connected PV segments 11 produces an output voltage (VSTRING) in the range of 288V to 612V. An inverter 15 converts VSTRING from dc to ac and has an output 16 for coupling to the electric power grid. There is also usually a disconnect switch 17 for shutting down the system 10.
Since the PV segments 11 are connected in series, the current (ISTRING) is the same in each segment. Therefore, when one segment is shaded (e.g., by a tree branch, or chimney) it acts like a bottleneck, restricting current flow in the entire string. The unshaded PV segments try to force current flow through the shaded segment, resulting in the shaded segment becoming reverse-biased. But a reverse-biased PV cell dissipates energy instead of producing energy, so the shaded segment gets hot, and can even be permanently damaged. The well known remedy is to include bypass diodes 14 that allow current to flow around the shaded PV segments, rather than through them. Thus, the bypass diodes 14 protect the PV segments from damage due to reverse bias, and also avoid a serious reduction in system 10 efficiency when the string is partially shaded.
A potential problem in PV systems, such as 10, is overheating in the bypass diodes 14. For example, assume the string current (ISTRING) is 11 Amps, but the short-circuit output current of one of the PV segments 11 is only 1 Amp, due to shading. This means the current in the bypass diode 14 connected in parallel to the shaded segment is 10 Amps. If the forward voltage drop of the bypass diode 14 is 0.5V at 10 A, then the heat dissipation in the bypass diode is 5 W. A typical junction box affixed to the back side of a solar power module contains three bypass diodes. Therefore, in this example, the heat dissipation inside the junction box could be up to 15 W. What is more, such junction boxes are typically relatively small, made of plastic that conducts heat poorly, and have no ventilation holes because they must keep out moisture. Consequently the junction temperatures of the bypass diodes 14 can easily exceed 180° C. under these conditions. Such high junction temperatures shorten the expected life span of the bypass diodes.
One solution for reducing heat dissipation, well known to those of ordinary skill in the art, is to use an active bypass diode circuit. There are many examples of such circuits in the prior art such as: U.S. Patent Application Publication number 2010/0002349 (La Scala, et al), U.S. Pat. No. 7,898,114 (Schmidt, et al), and U.S. Patent Application Publication number 2009/0014050 (Haaf).
FIG. 2 is a high level block diagram that is typical of such prior art, showing an active bypass diode circuit 20 comprising: a bypass diode 21, a switch 22, a control circuit 23, a charge pump circuit 24, and a capacitor 26. When the PV segment 11 is partially shaded, current initially flows through the bypass diode 21. The control circuit 23 senses the resulting positive voltage from the diode's anode 13 to it's cathode 12, and closes the switch 22, thereby reducing power dissipation. When the PV segment 11 is unshaded, the control circuit 23 senses a negative anode-to-cathode voltage and opens the switch 22 again. The switch 22 is typically a metal-oxide-semiconductor field-effect transistor (MOSFET) with an on-resistance of about 5 mΩ. At 10 A, the power dissipation in a junction box containing three such active bypass diode circuits would be approximately 1.5 W, or about 90% less than with conventional bypass diodes.
Another problem with a conventional PV system, such as 10, is safety for installer personnel and firefighters. The electrocution hazard for a PV array is much greater than with ordinary 120V ac wiring found in the typical American home, mainly because the voltage is dc. But in addition to the increased danger of electrocution, there is also a lesser-known danger from arc flashes. An electrical arc is an ongoing plasma discharge caused by current flow through a normally non-conductive medium, such as air. With ac power systems, you get a spark when you open a circuit—a spark is a very brief arc—but with dc power systems, the arc can be continuous, and therefore the temperature quickly reaches thousands of degrees, which is why electrical arcs are used for welding. According to OSHA there are approximately 2000 injuries in the U.S. each year related to arc flashes. Statistics on how many of these injuries are in the PV solar industry are not available at the time of this writing, but it can be assumed the number is not insignificant because solar workers typically have much less safety training and equipment compared to their counterparts at electrical utilities.
Systems for suppressing, or mitigating arc flashes in PV solar systems already exist. For example, many companies now make products called dc power optimizers that automatically shut off when a cable is disconnected. However, these products perform many other functions besides arc flash mitigation, and are relatively very expensive.
Therefore, there is a need in the solar power industry for a low-cost active bypass diode circuit and solar power module that not only reduces heat dissipation under partial shading conditions, but also mitigates arc flash hazards for installers and firefighters.